1. Field of the Invention
The present invention relates to an optical system capable of correcting an image position (also referred to as an image position correcting optical system hereinafter) and, more particularly, to an objective lens system suitable for SLR cameras (Single-Lens Reflex cameras), electronic cameras, and video cameras.
2. Related Background Art
In the present specification, an operation for correcting any variation of an image position caused by a focusing operation, a camera shake, or the like by displacing a predetermined lens group in a direction substantially perpendicular to the optical axis will be referred to as "image position correction". In particular, an operation for correcting any variation of an image position caused by a vibration of an optical system such as a camera shake, or the like by displacing a predetermined lens group in a direction substantially perpendicular to the optical axis will be referred to as "vibration reduction" or "vibration correction".
As a conventional image position correcting optical system of this type, an optical system disclosed in U.S. Pat. No. 4,978,205 is known.
The optical system disclosed in U.S. Pat. No. 4,978,205 has a large total lens length (the distance from a lens surface, closest to the object side in the optical system to the image plane), and is large in size.
As described above, the conventional image position correcting optical system is inconvenient for carrying due to its large total lens length, i.e., has poor portability. Since the optical system is large in size, it is heavy and hard to support (hold). Therefore, although the optical system originally aims at reducing camera shake, the camera shake increases since the optical system is hard to hold, resulting in a large variation of the image position.
In the optical system disclosed in U.S. Pat. No. 4,978,205, an aperture stop is arranged at a position closest to the image side in the optical system. Therefore, as a problem unique to a behind stop, an insufficient improvement of the marginal light amount ratio upon stopping-down of the aperture stop becomes undesirably outstanding. In the optical system disclosed in U.S. Pat. No. 4,978,205, since a lens group for attaining a focusing operation and a lens group for correcting an image position have large effective diameters, the size of a lens barrel increases in its radial direction, resulting in poor operability in a hand-held photographing operation.